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50-minute se
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ACTIVITY OVERVIEW
LAB
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94
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Comparing Colors
ORA
T
Students explore light by investigating the colors of the visible spectrum. They first
observe how a diffraction grating splits white light into its component colors. Then
they investigate the energy levels of the different colors of white light through the use
of a phosphorescent material.
KEY CONCEPTS AND PROCESS SKILLS
(with correlation to NSE 5–8 Content Standards)
1.
Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical.
(PhysSci: 3)
2.
Energy is transferred in many ways. (PhysSci: 3)
3.
The sun’s energy arrives as light with a range of wavelengths, consisting of visible
light, infrared, and ultraviolet radiation. (PhysSci: 3)
4.
Scientists use appropriate tools and techniques to gather, analyze, and interpret
data. (Inquiry: 1)
KEY VOCABULARY
evidence
frequency
visible light spectrum
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Activity 94 • Comparing Colors
MATERIALS AND ADVANCE PREPARATION
For the teacher
1
Scoring Guide: ANALYZING DATA (AD)
1
Scoring Guide: EVIDENCE AND TRADE-OFFS (ET)
1
diffraction grating
*
1
flashlight
*
1
white surface or wall
For each pair of students
*
1
Phospho-box
1
card with star-shaped cutout
1
colored-film card
1
timer
1
set of colored pencils (red, orange, yellow, green, blue, and purple)
(optional)
For each student
1 Scoring Guide: ANALYZING DATA (AD) (optional)
1 Scoring Guide: EVIDENCE AND TRADE-OFFS (ET) (optional)
*Not supplied in kit
Masters for all Scoring Guides can be found in Teacher Resources III: Assessment.
Practice using diffraction grating to diffract white light into the visible spectrum for
the demonstration described in Step 1 of the Teaching Suggestions.
For best results, conduct this activity inside.
TEACHING SUMMARY
Getting Started
1.
Introduce colors of the visible spectrum.
Doing the Activity
2.
Investigate the frequencies of visible light colors.
3.
Review the definition of evidence.
Follow-Up
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4.
Introduce the relationship between color and frequency.
5.
(AD ASSESSMENT) Students analyze transmission graphs.
6.
(ET ASSESSMENT) Identify the trade-offs of different lenses.
Comparing Colors • Activity 94
BACKGROUND INFORMATION
Refraction
Refraction occurs when a wave propagating through one medium (or through a vacuum, in the case of light) encounters the interface of another media at an angle. This
results in a change in the angle of propagation of the wave as it travels across the
interface. For example, the frequency of a wave is determined at the source and is
fixed after the wave has left the source. However, the speed and the wavelength can
change depending on the medium through which the wave travels. There is, as a
result, a differential slowing of the wavefront that causes the light to refract, or
change direction. In the case of white light, different frequencies of the light are redirected at slightly different angles, producing the light spectrum that we observe as a
rainbow.
Diffraction Grating
A diffraction grating is a tool that diffracts the light that passes through it. It has a
similar effect on light as a prism that refracts white light into the visible spectrum,
although the sequence of the spectrum is reversed. The mechanism for the separation
of light into a spectrum via diffraction is different from that when light is refracted; in
diffraction, the light bends around small obstacles that are roughly the same size as
the wavelength of the light. A diffraction grating is a transparent or reflective film
with parallel thin rulings on it, which cause this bending or dispersion of light. The
direction in which the grating disperses light depends on the spacing of the grating
and the wavelength of light. Diffraction gratings are commonly found in spectrometers and monochromators. Likewise, the tracks of a CD are separated by the distance
of an ordinary lab diffraction grating and will produce a separation of white light.
Visible Light Spectrum
The visible light spectrum is that portion of the electromagnetic spectrum that is visible to the human eye and is perceived as color. It ranges in wavelength from about
400 nm (violet) to 700 nm (red). The boundaries are somewhat hard to distinguish as
the colors blend and the outermost regions blend into ultraviolet and infrared. The
table below shows the approximate range for each color. It also shows that the bandwidth for each color is not evenly distributed, with red having the widest wavelength
range and yellow the thinnest.
Color
Wavelength (nm)
Wavelength range (nm)
Violet
380–450
70
Blue
450–495
45
Green
495–570
75
Yellow
570–590
20
Orange
590–620
30
Red
620–750
130
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Activity 94 • Comparing Colors
TEACHING SUGGESTIONS
GETTING STARTED
1.
Introduce the colors of the visible
spectrum.
Introduce the activity by asking, Who has ever seen
a rainbow? It is likely that all of your students have
seen one. Next ask, What causes the colors of a rainbow? Record their suggestions on chart paper, a
board, or smartboard. Make sure students are
aware that for a rainbow to form, there must be
water droplets in the air and sunlight to pass
through the water droplets. This phenomenon is
called refraction.
Hold up the diffraction grating up and explain that
this film with grating on it can also make a rainbow
by splitting up the white light that passes through
it. Point out that the grating works by a different
mechanism than a prism. Whereas refraction is a
result of the different frequencies of light being redirected through the glass, diffraction is a result of
white light being spread out when it is transmitted
through very fine slits. Conduct a demonstration
that shows the visible light spectrum by holding the
grating about 6” in front of a light source, such as a
flashlight. Move the grating around a bit until the
diffracted light is projected onto a white surface,
such as a wall or paper.
Point out that the rainbow is not formed directly in
front of the grating, but is instead angled upward or
to the side of it. Ask students, What does this tell you
about the light that goes through the grating? If necessary, explain that this is evidence that the grating
splits up the white light. Then explain that due to
the shape of a grating, the incoming white light is
separated into its component colors and it appears
in its various colors.
Procedure Step 2
Students should list the following colors in either
ascending or descending order: red, orange, yellow,
green, blue, violet. They might also mention the
color indigo. Scientists no longer classify indigo as a
color in the visible light spectrum because it is a rel-
F-36
atively narrow band of color that is transitional
between blue and violet. The brightest colors are
often seen in the middle of the spectrum, in the yellow area where our eyes are most sensitive.
In reviewing responses to Procedure Step 2, emphasize that the order of the visible light spectrum
always shows in the same order of red, orange, yellow, green, blue, violet or vice versa, regardless of
how it is diffracted or refracted. Students may surmise that this is because each color has some
unchanging characteristic that provides the rank.
Procedure Step 3
Sample Student Response
The colors blend from one to the next with a smooth
transition between them.
In reviewing responses to Procedure Step 3, reinforce the idea that the visible spectrum is continuous from red to violet. Although students are not
familiar with the entire electromagnetic spectrum
at this point, the evidence for answering this question foreshadows later activities where students
learn about the continuous nature of the electromagnetic spectrum.
Procedure Step 4
Sample Student Response
Yellow appears the brightest, with the colors on the
outside of it, orange and green, the next brightest.
Closely followed is red, with blue and violet the
least bright.
Explain to students that the sun does not give off
more yellow light and less red or violet (in fact, it
gives off more green than any other color), but
rather, our eyes are more sensitive to the color yellow than to the other colors. These observations
provide some evidence to students that sunlight has
unequal intensity of different colors, that our eyes
are sensitive to those colors, or both.
To prepare students for Part B: “Colored Light,” ask
Do you think that each color of light contains the
same amount of energy? A common response is that
yellow contains the most energy because it looks the
brightest. Do not tell students at this time that this is
Comparing Colors • Activity 94-
incorrect, but do ask a follow-up question, such as,
What other observations or measurements could
help answer the question of what color contains the
most energy? Students tend to suggest taking the
temperature of the colors. Explain that Part B of this
activity should provide some additional evidence to
help answer the question of which color has relatively more energy.
DOING THE ACTIVIT Y
2.
Investigate the frequencies of visible light
colors.
Review how the colored-film card works before starting Part B. In this investigation, each colored film
isolates a single color of light. Explain that colored
film doesn’t refract light like a refraction grating or
a water drop to separate the colors in white light.
Instead, it only allows one color to be transmitted
through the film and come out the other side.
A common student misconception is
that color is transferred to the light
from the colored film, much like paint
is put onto an object. Make it clear to students that
the films do not add any color to white light.
With the Phospho-box, students should observe
that only blue and violet light cause the phosphorescent strip to glow, even when they double the
exposure time. Since the phosphorescence in the
strip is triggered by a threshold energy, this is evidence that the blue and violet lights have more
energy than the other colors. For improved results,
students should hold the boxes closer to the light
source when exposing them.
Procedure Step 10
Sample Student Response
Violet was the brightest, then blue. The other colors
did not trigger the phosphorescent strip. The violet
seemed equally as bright as when the card was not
used.
Next ask, Why don’t all of the colors make the strip
glow? Some students may suggest that not all colors
of light carry energy. Make it clear that all colors
carry energy, but each color carries a different
amount. Each color is due to a wave with a slightly
different frequency. Only some frequencies carry
enough energy to cause the phosphorescent material in the strip to glow. Point out that the colors that
make the strip glow (blue and violet) are found
right next to each other in the rainbow. This gives
some evidence that higher frequencies (and, therefore, energy) of a light wave are related to its position in the spectrum. For visible light, violet has the
highest frequency, and red has the lowest. The rest
of the colors are in between, according to their position in the rainbow. The phosphorescent strip has a
threshold energy. Any energy equal to or greater
than the threshold will make the strip glow. The
threshold energy corresponds to the frequency
delivered to the strip by blue light.
In Procedure Step 15, students should find that
when they double the time duration, the results are
similar. This indicates that the phosphorous strip is
sensitive to the frequency of the light and not the
total exposure.
3. Review the definition of evidence.
Although the term evidence is used previously in
the unit, Analysis Question 4 provides an opportunity to formally present what is and isn’t scientific
evidence. Review the definition of evidence provided in the Student Book. Explain that scientists
collect information (data) with various tools and
strategies, including observation and experimentation. Tell students that they will now use the data
they collected from the film experiment to decide if
it gives information about what is damaging Tía
Ana’s eyes. The consideration of evidence is a key
step in scientific reasoning and decision-making.
Throughout this unit, and throughout Issues and
Physical Science, students will collect and analyze
information, which may become evidence to support or refute claims.
It is important that students be able to distinguish
evidence from opinion. Evidence is information
that supports a claim. Opinion is the view someone
takes about a certain issue based on his or her own
judgment, often without the support of factual evidence. An informed opinion may be based on evidence; however, another person may have a
F-37
Activity 94 • Comparing Colors
different opinion based on the same evidence. To
distinguish evidence from opinion it is helpful to
determine if the information is reproducible, meaning could someone else gather similar information
under similar circumstances? If the answer is yes,
the information is not opinion and is likely to be evidence.
Students should also learn to distinguish supposition from inference. A supposition is a claim based
on an incomplete set of evidence. In contrast, an
inference is a logical conclusion arrived at by an
examination of a complete set of evidence.
When making a decision based on evidence, one
must be critical of the source, quality, and quantity
of evidence available. Review with students that scientific conclusions are based on evidence, and
biased or insufficient evidence compromises the
validity of these conclusions. The criteria for quality
evidence may vary among the scientific disciplines.
However, evidence is generally considered of higher
quality if it is obtained through systematic investigation and is reproducible, meaning another investigation under the same set of circumstances would
obtain similar data. Additionally, the greater the
quantity of high-quality evidence that can be provided the stronger a case is in support of, or against,
a claim. Criteria for quantity also vary, but might
include the sample size or number of trials in an
experiment, the number of observations that support a conclusion, or the availability of multiple
lines of evidence that lead to the same conclusion.
Scientific conclusions should logically follow the
evidence collected, and should not be overly generalized beyond the context of the investigation.
FOLLOW-UP
4.
Introduce the relationship between color
and frequency.
To relate the results of the Phospho-box procedure
to the story of Tía Ana, ask students which color of
visible light is more likely to damage eyes due to its
higher energy. Students should respond that the
violet light is most likely to be damaging. In fact, it
is not the violet light that is damaging but ultravio-
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let just beyond the violet frequencies. Students will
be introduced to ultraviolet in subsequent activities,
and so use this discussion of higher and lower energies to explore the frequencies of the various colors.
It may be helpful to present a diagram like the one
below that shows the relative frequencies through
the visible spectrum. For the same intensity of light,
those light waves that are higher frequency will also
have higher energy than light waves of lower frequency. Student’s evidence from the activity supports this idea. Let students know that in the next
activity, they will consider waves that are of a frequency higher than violet light, such as those that
could damage Tía Ana’s eyes.
R
Longer wavelength
Lower frequency
Lower energy
5.
O
Y
G
B
V
Shorter wavelength
Higher frequency
Higher energy
(AD ASSESSMENT) Students analyze
transmission graphs.
Review Analysis Question 6a in which students are
asked to identify the lenses that block out highenergy wavelengths. Model how to read one of the
graphs, and describe how it shows transmission at
different wavelengths and also provides an overall
sunlight percentage that is transmitted. This question is a good opportunity for assessing student’s
ability to not only read the graphs, but to apply the
concept of selective transmission introduced by the
colored films. Selective transmission is further
investigated in the next activity.
Students’ written work from Analysis Questions 6a
may be scored with the ANALYZING DATA (AD) Scoring Guide. A complete and correct response is
shown in Suggested Answers to Analysis Questions.
Comparing Colors • Activity 946.
(ET ASSESSMENT) Identify the trade-offs of
different lenses.
Analysis Question 6b introduces the concept of a
trade-off. This concept is empahsized in the unit’s
last activity, Activity 99, “Personal Protection Plan,”
but is introduced here as part of the Evidence and
Trade-offs assessment. Provide all students with an
EVIDENCE AND TRADE-OFF (ET) Scoring Guide, and ask
them to keep it with their science notebooks, as they
will refer to it several times in this unit, and
throughout Issues and Physical Science. Explain to
the class that you will apply the ET Scoring Guide to
provide feedback on the quality of their work. As a
class, discuss what a Level-3 response would
include. In this case students’ responses should
include a clear undertanding of each lens based on
the graphs and a discussionof the trade-offs of making their choice. You may develop a Level-3 exemplar with the class or share with students the
Level-3 response shown in the Suggested Answers to
Questions section. Point out the elements that make
the example a Level-3 response, and discuss how a
Level 1 and a Level 2 differ. Ask students for ideas
about how to improve the Level-3 response to make
it a Level 4. For more information see Teacher
Resources III: Assessment.
asked, “Paper or plastic?” at a store checkout
counter, most shoppers make the choice quickly. But
there are several trade-offs attached to choosing
paper or plastic. A shopper who chooses paper over
plastic may do so to avoid generating plastic waste
or using up petroleum resources. In requesting the
paper bag, though, they are contributing to other
environmental problems, such as increased water
and energy use, and the higher amounts of solid
waste and CO2 emissions associated with making
paper bags. Neither choice is particularly beneficial
for the environment, and both choices have a downside. Identifying the trade-offs helps clarify the reasoning that is being applied to make a decision.
To further explore trade-offs, brainstorm with the
class a list of decisions they make every day that
involve trade-offs. Choose one and talk through the
associated trade-offs of deciding one way or
another. This practice will familiarize students with
ways of identifying and considering trade-offs in
this and subsequent activities.
SUGGESTED ANSWERS TO QUESTIONS
1.
As a class, discuss what a Level-3 response would
include. In this case it should include a discussion
on the benefits and trade-offs of different sunglass
lenses.
The star-shaped cutouts provide a control so that
we can see what white light, which contains all
of the colors, does to the strip in the Phosphobox. Then we can compare the effects of each
color to the effect of white light.
One of the goals of Issues and Physical Science is to
teach students that
• decisions often involve trade-offs.
• identifying trade-offs involves analyzing evidence.
Explain to students that in this unit they will make
several decisions about what types of protective
materials or equipment to use in various situations.
In a decision involving trade-offs something is given
up to gain something else. Since many decisions
involve trade-offs, students should understand that
a perfect choice is often not possible. It is possible,
however, to recognize and analyze the trade-offs
associated with each decision. For example, when
What is the purpose of the card with the
star-shaped cutout?
2.
How do you think the colored-film card
changes the white light into colored light?
Answers will vary. Even though you discussed
this earlier, some students will indicate that the
color is transferred to the light from the colored
film. Although it is hard to provide convincing
evidence to the contrary, make clear that this is
not what is happening, and that instead, each
film is acting as a filter and letting only one color
through and is blocking the rest of the colors.
Transmission, reflection, and absorption will be
further investigated in the next activity.
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Activity 94 • Comparing Colors
3.
Why do you think only some colors make
the strip on the bottom of the Phospho-box
glow? Explain.
Answers will vary. Lead students to the understanding that light carries energy, and when it is
absorbed by a material that energy is transferred to the material. In the case of the material
that makes up the strip on the box, if light with
enough energy is absorbed, the energy is then
reemitted as the glowing light. Point out that
glow-in-the-dark toys are made of a similar kind
of material.
4. Is there enough evidence, or information that supports or refutes a claim, that supports the idea that
the higher-energy colors of white light are damaging
Tía Ana’s eyes?
There is evidence that there is a range of energy
carried by white light, with some colors (blue,
violet) having more energy than others (red,
orange). However, there is no evidence that supports the idea that the relatively higher energy
in some colors is enough to damage Tía Ana’s
eyes.
5. Using the graph below, why do you think sunlight is
yellow instead of purple?
The graph shows that the largest percentage of
color in the visible light spectrum is in the middle of the spectrum or near the yellow frequency.
6.
Sunglass lenses are an example of a material
that blocks some white light and some other highfrequency light that is harmful to the eyes. Examine
the transmission graphs about three pairs of sunglasses below.
a. (AD ASSESSMENT) Which lens has the best highenergy protection for the eyes? Explain how you
decided.
Answers will vary. Students could make their
choices on price and/or ultraviolet protection, but many may be more concerned with
the style and color of the frames and lenses.
Encourage students to be clear about what is
influencing their choice. Look for responses
that specifically identify the data on the
graphs as influencing their choice.
F-40
Level 3 Response
Lens 1 provides the best protection. This can
be determined because its graph goes down
close to zero for the ultraviolet wavelengths.
Lens 3 is nearly as good because it lets only a
little more of the UV through, and most of
the light is transmitted more evenly (except it
blocks more blue and violet). So Lens 1 must
look red and Lens 3 looks blue. All three of
the lenses provide a lot of blocking in the UV
frequencies. In fact, both lenses 1 and 3
transmit only 30% of the light. They are all
better protection than if you were not wearing sunglasses.
b. (ET ASSESSMENT) The price for each pair of sunglasses is shown below. Which pair would you
buy and why? Describe any trade-offs you made
in your choice.
Lens 1: $80
Lens 2: $10
Lens 3: $20
Answers will vary. Students could make their
choices on price and/or ultraviolet protection, but some may be more concerned with
the style and how dark the lenses appear.
Encourage students to be clear about what is
influencing their choice. Look for responses
that specifically identify the trade-off being
made, such as higher cost for effectiveness.
Level-3 Response
Although Lens 1 is the best protection, I
wouldn’t buy it because it is the most expensive one. Lens 2 provides nearly as much UV
protection and is only 1/8th the cost. I would
choose this lens because it is cheaper than
both of the other lenses and has similar protection to Lens 1. In fact, the cheaper Lens 2
has better protection than Lens 3. The tradeoff is that it is a very light lens with 60% of
the light going through it, and I prefer a dark
lens. It is a better trade-off than selecting
Lens 3, however, because I would not like to
wear a red lens at all.